Some of the oldest known rock formations, formed over 3,700 million years ago (mya), include banded iron layers.[1] The banded layers are a common feature in sediments for much of the Earth's early history.[2]

The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosyntheticcyanobacteria. This combined with dissolved iron in Earth's oceans to form insoluble iron oxides. These precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve: the banding is assumed to result from cyclic variations in available oxygen.

It is assumed that the Earth started out with vast amounts of iron dissolved in the world's acidic seas.[4]

Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides.

At the tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition.[5]